[Bug 212183] New: st read statistics inaccurate when requested and physical block mismatch

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https://bugzilla.kernel.org/show_bug.cgi?id=212183

            Bug ID: 212183
           Summary: st read statistics inaccurate when requested and
                    physical block mismatch
           Product: IO/Storage
           Version: 2.5
    Kernel Version: 5.3.1
          Hardware: All
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: low
          Priority: P1
         Component: SCSI
          Assignee: linux-scsi@vger.kernel.org
          Reporter: etienne.mollier@cgg.com
        Regression: No

Created attachment 295769
  --> https://bugzilla.kernel.org/attachment.cgi?id=295769&action=edit
st.c patch working around stats issue when blocks size mismatch

Greetings,

when reading from tape with requested blocks larger than physical, statistics
go wrong as using the requested size for the calculation, instead of the actual
size of the block returned.  So, when running `dd if=/dev/st0 bs=10240
of=/dev/null`, a tool such as `tapestat` will work out the bandwidth using the
bs=10240.  However, if the block on tape was of size 1024, then the metric
would go wrong by a factor 10.

Most configurations won't notice, as tapes are usually fixed size block,
typically for backup use case.  However on our end, the problem exacerbates due
to reading field acquisition cartridges with varying block size.  We are
currently working this around with the patch in attachment.

While we've observed the issue on a somewhat old kernel revision, checking out
the "master" branch of the linux tree, we believe this is present since
introduction of the capability in Linux 4.2, and likely to reappear with more
recent kernel revisions if left as-is.

Kind Regards,
Étienne.

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